Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Millions	Total	Common Stock [Member]	Additional Paid-in-Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income [Member]	Accumulated Translation Adjustment [Member]	Treasury Stock [Member]
Balance at Sep. 29, 2012	$ 2,961.0	$ 2.6	$ 5,396.7	$ (2,443.6)	$ 6.8		$ (1.5)
Balance (in shares) at Sep. 29, 2012		265,635					219
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options	65.7	$ 0.1	65.6
Exercise of stock options (in shares)		4,786
Issuance of common stock to employees upon vesting of restricted stock units, net of shares withheld for employee taxes	(12.3)		(12.3)
Issuance of common stock to employees upon vesting of restricted stock units, net of shares withheld for employee taxes (in shares)		1,117
Issuance of common stock under the employee stock purchase plan	8.0		8.0
Issuance of common shares under the employee stock purchase plan (in shares)		498
Stock-based compensation expense	52.4		52.4
Excess tax benefit from employee equity awards	5.9		5.9
Equity component related to convertible notes, net of taxes	20.0		20.0
Net income (loss)	(1,172.8)
Foreign currency translation adjustment	(1.4)				(1.4)
Adjustment to minimum pension liability, net	0.1				0.1
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	0.0
Unrealized losses on marketable securities	12.1				12.1
Balance at Sep. 28, 2013	1,941.5	$ 2.7	5,536.3	(3,616.4)	20.4		$ (1.5)
Balance (in shares) at Sep. 28, 2013		272,036					219
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options	70.6	$ 0.1	70.5
Exercise of stock options (in shares)		4,697
Issuance of common stock to employees upon vesting of restricted stock units, net of shares withheld for employee taxes	(9.8)		(9.8)
Issuance of common stock to employees upon vesting of restricted stock units, net of shares withheld for employee taxes (in shares)		846
Issuance of common stock under the employee stock purchase plan	10.9		10.9
Issuance of common shares under the employee stock purchase plan (in shares)		612
Stock-based compensation expense	49.5		49.5
Excess tax benefit from employee equity awards	0.8		0.8
Net income (loss)	17.3
Foreign currency translation adjustment	13.3				13.3
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.0
Adjustment to minimum pension liability, net	(1.3)				(1.3)
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	0.0
Retirement of treasury shares	0.0			(1.5)			$ 1.5
Retirement of treasury shares (in shares)		(219)					(219)
Unrealized losses on marketable securities	(3.2)				(3.2)
Balance at Sep. 27, 2014	2,063.0	$ 2.8	5,658.2	(3,600.6)	2.6		$ 0.0
Balance (in shares) at Sep. 27, 2014		277,972					0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options	$ 57.3		57.3
Exercise of stock options (in shares)	3,000	3,036
Issuance of common stock to employees upon vesting of restricted stock units, net of shares withheld for employee taxes	$ (12.9)		(12.9)
Issuance of common stock to employees upon vesting of restricted stock units, net of shares withheld for employee taxes (in shares)		949
Issuance of common stock under the employee stock purchase plan	12.0		12.0
Issuance of common shares under the employee stock purchase plan (in shares)		538
Stock-based compensation expense	54.6		54.6
Excess tax benefit from employee equity awards	7.6		7.6
Equity component related to convertible notes, net of taxes	(216.9)		(216.9)
Net income (loss)	131.6
Foreign currency translation adjustment	20.6				20.6
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	17.4				7.8	$ 9.6
Adjustment to minimum pension liability, net	(0.2)				(0.2)
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	3.9				(3.9)
Unrealized losses on marketable securities	(9.8)				(9.8)
Balance at Sep. 26, 2015	2,079.2	$ 2.8	$ 5,559.9	$ (3,469.0)	$ (14.5)		$ 0.0
Balance (in shares) at Sep. 26, 2015		282,495					0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other than temporary impairment	$ 7.8